The invention relates to construction of a pump-actuating mechanism for a high-capacity reciprocating hydraulic pump, particularly having a discharge capacity of 10 liters/minute or more and at a pressure exceeding 1000 bar. The plunger is exposed to liquid in the pump chamber, between check valves at the inlet and outlet of the chamber.
Pump-plunger constructions of this type are known, wherein the plunger is a cylindrical rod, driven at one end and guided at its other end by the bore of a plunger housing. The plunger housing is open to the pump chamber at the guided end; and axially behind the region of plunger guidance, the bore expands to an elongate cylindrical counterbore, within which a sealing ring of relatively large axial length is axially compressed between two solid back-up rings which add substantially to the relatively large axial length. A helical spring is also contained within the counterbore, being interposed between the forward back-up ring and the sealing ring, to compressionally bias or preload the sealing ring against the rear back-up ring, whereby the sealing ring is axially squeezed to enlarge its outer diameter into sealed relation with the counterbore and to reduce its inner diameter into sealed engagement with the outer cylindrical surface of the plunger. When the plunger leaves its retracted and sealed position in a driven forward stroke, the biasing action of the spring is momentarily reduced and the rear back-up ring transiently undergoes an axial displacement along the plunger.
It has been found, however, that especially at high pumping capacities in the order of a multiple of 10 liters/min. the known pump-actuating mechanism ceases to operate satisfactorily, as soon as discharge pressures are required to be greater than 1000 bar, and in particular greater than 1400 bar.